1. Field of the Invention
The conventional fluid bed boilers normally involve a shallow bed of coarse particles, either inert or sulfur-accepting stone, fluidized by air at a rate sufficient to bring about vigorous fluidization and provide efficient combustion. Arrays of heat exchange tubes are located within the bed in order to exploit the favorable heat transfer characteristics of fluidized beds.
The nature of the shallow bed and large particles which are employed create numerous engineering problems in developing a satisfactory design. The design should provide for uniform distribution of air as well as good solid mixing across the bottom of the bed. With coarse material, this inevitably means a distributor with a large number of holes per unit bed area (or its equivalent if actual holes are not used). The distributor should incur a minimal pressure drop compatible with its functions and should offer the promise of durability and minimum maintenance.
Not only must the air be uniformly distributed, more importantly there must be quick and uniform lateral distribution of the coal over the entire cross section of the bed. This requires feeding of the coal through a large number of inlets, and a complex system of fuel distribution.
Because of the need to maintain a temperature around 1500.degree. F for optimum capture of sulfur, the conventional fluid boiler is poorly suited for following load. Reducing the air and coal rates would inevitably cause an appreciable drop in bed temperature with attendant loss of sulfur capture efficiency.
Furthermore, the conventional fluid boiler often gives rise to a substantial carryover of unburnt char. Combustion of some of this char in the freeboard above the bed, where the concentrations of sulfur-accepting stone and oxygen are low, results in escape of sulfur dioxide and carbon monoxide. For high carbon utilization, the carbon in the entrained dust and the carbon monoxide will require additional burn up zones for their consumption.
Calcium to sulfur ratios required to achieve the desired level of sulfur reduction are generally high. This is a consequence of the relatively poor contacting between gas and solid that the shallow bed of coarse particles affords. Finally, because of the relatively low coal processing throughput of the conventional fluid bed boiler - generally below 100 lbs of coal per hour per unit area of bed cross section - utility size installations may entail cumbersome and complex designs.
2. Description of the Prior Art
U.S. Pat. No. 2,498,088 describes a fast bed catalytic cracker. Fast fluidized beds are described in Reh, Chem. Eng. Progr. 67 (2), 58 (1971); and Yerushalimi, et. al., Ind. Engr. Chem. Process, Design and Development 51, 47 (1976). Multiple beds are described in Elliott, IMech E/VDI Joint Energy Convention, Dusseldorf, May 57, 1975.
Patents of interest employing multiple beds include U.S. Pat. Nos. 2,700,592, 3,763,830, 3,840,353, 3,868,993, 3,884,649, and 3,902,462.